Boss VE-20 Vocal Effects Processor

The Boss VE-20 is one versatile pedal! It combines effects, voice doubling (2, 3, and 6 parts) harmonization (3rds, 5ths, or both 3rds and 5ths in multiple inversions), 38 seconds of looping capability, and real-time pitch correction (subtle or harsh) into a single unit!

Here's a demo from Sweetwater Sound showing what the VE-20 can do when combined with a theremin: